Capacitorless dram on bulk silicon

ABSTRACT

A method of forming capacitorless DRAM over localized silicon-on-insulator comprises the following steps: A silicon substrate is provided, and an array of silicon studs is defined within the silicon substrate. An insulator layer is defined atop at least a portion of the silicon substrate, and between the silicon studs. A silicon-over-insulator layer is defined surrounding the silicon studs atop the insulator layer, and a capacitorless DRAM is formed within and above the silicon-over-insulator layer.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/450,661, filed on Jun. 8, 2006, issued Apr. 14, 2009 as U.S. Pat. No.7,517,744, which is a divisional of U.S. application Ser. No.11/148,853, filed on Jun. 8, 2005, the entire disclosure of which ishereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to a localized silicon-on-insulator(“SOI”) semiconductor design, and, more particularly, to the creation oflocalized oxide in the array of dynamic random access memories (“DRAM”).

BACKGROUND OF THE INVENTION

Use of a silicon-on-insulator, or SOI, substrate generally enables themanufacture of typical circuit elements over an insulator, such asoxide. In one application, capacitorless DRAMs may be formed on SOI. Useof the SOI design versus a traditional silicon substrate increases thefloating body effect for the access transistors of these capacitorlessDRAMs, yielding far more effective storage. The programming of thefloating bodies in such DRAMs may be done either by impact ionization(“II”) or by gate induced drain leakage (“GIDL”). The sensing isnon-destructive and is done using a resistance or current sensing methodat a lower voltage. Further description of capacitorless DRAM via GIDLmay be found in Yoshida et al., A Design of a Capacitorless 1 T-DRAMCell Using Gate-induced Drain Leakage (GIDL) Current for Low-power andHigh-speed Embedded Memory, Technical Digest—International ElectronDevices Meeting 2003, pp. 913-916 (IEEE Cat. No. 03CH37457, 2003), thecontents of which are incorporated herein in its entirety.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a method of formingcapacitorless DRAM over localized silicon-on-insulator is disclosed. Themethod comprises the following steps: A silicon substrate is provided,and an array of silicon studs is defined within the silicon substrate.An insulator layer is defined atop at least a portion of the siliconsubstrate, and between the silicon studs. A silicon-over-insulator layeris defined surrounding the silicon studs atop the insulator layer, and acapacitorless DRAM is formed within and above the silicon-over-insulatorlayer.

According to another embodiment of the invention, a method of forming amemory chip is disclosed. The method comprises the following steps: Aperiphery region and a memory array region are defined on the memorychip. At least one silicon-over-insulator region is formed in the memoryarray region, without forming a silicon-over-insulator region in theperiphery region. At least one capacitorless DRAM is formed on andwithin the at least one silicon-over-insulator region.

According to another embodiment of the invention, a memory device isdisclosed. The memory device comprises a source and a drain. The memorydevice further comprises a floating body formed between the source andthe drain, the floating body defined within a localizedsilicon-over-insulator. The memory device further comprises a gateadjacent the floating body.

According to another embodiment of the invention, an integrated circuitis disclosed. The integrated circuit comprises a periphery region, andan array region. At least one localized silicon-over-insulator is formedwithin the array region. The integrated circuit further comprises asource and a drain formed within the array region. A floating body isformed between the source and the drain within the at least onelocalized silicon-over-insulator. The integrated circuit furthercomprises a gate adjacent the floating body.

According to another embodiment of the invention, a system is disclosed.The system comprises a source, and a first drain and a second drain. Thesystem further comprises a first floating body formed between the sourceand the first drain, and a second floating body formed between thesource and the second drain, the floating bodies defined within alocalized silicon-over-insulator. The system further comprises a firstgate adjacent the first floating body, and a second gate adjacent thesecond floating body.

According to one embodiment of the invention, a method of operating acapacitorless DRAM is disclosed. The method comprises the followingsteps: A floating body is placed in a first state, and the first stateis detected by measuring a first current at a source of thecapacitorless DRAM. The floating body is defined within a localizedsilicon-over-insulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a portion of a memorydevice on which a first step of a process for forming localized SOIaccording to one embodiment of the present invention has been performed.

FIG. 2 illustrates the memory device of FIG. 1 on which a second step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 3 illustrates the memory device of FIG. 1 on which a third step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 4 illustrates the memory device of FIG. 1 on which a fourth step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 5 illustrates the memory device of FIG. 1 on which a fifth step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 6 illustrates a capacitorless DRAM built over the localized SOIsubstrate of FIG. 5.

FIG. 7 is a partial top-down plan view of the capacitorless DRAM of FIG.6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the preferred embodiments of the present invention illustratelocalized SOI in combination with capacitorless DRAM, it should beunderstood that these methods of forming localized SOI may beincorporated into the fabrication of other integrated circuits as well.In addition, while the following methods are described in terms ofparticular DRAM fabrication techniques, as would be well known to thoseof skill in the art, such techniques may be replaced by other methods offabricating and modifying semiconductor materials.

SOI is typically created by a uniform layer transfer. Thus, in order tomanufacture capacitorless DRAM over SOI, for example, the entire surfaceof the array and periphery of the memory device incorporates a SOIsubstrate. However, while the SOI is desirable within the array, wherethe insulator enhances floating body effects, it adversely affectsperformance of the chip in the periphery.

Moreover, the creation of SOI via layer transfer is a difficult,time-consuming and expensive process. The fusion of different siliconand insulative layers poses many technical problems and must beperformed at particular temperatures under particular conditions.

Therefore, there is a need in the art to create localized areas of SOIby conventional semiconductor fabrication techniques while leaving therest of the chip unmodified. There is a further need for a method ofmaking capacitorless DRAM using bulk silicon by conventional DRAMfabrication techniques. Thus, the advantages of capacitorless DRAM mightbe achieved without the expense and difficulty of creating SOI via layertransfer.

FIGS. 1 through 5 illustrate one method for forming localized SOIinexpensively and efficiently. According to this method, the SOIstructure(s) may be formed solely within the array of a memory chip,leaving the periphery to be formed on and within a typical semiconductorsubstrate.

Although not shown in any figure, the following method of forminglocalized SOI may be performed using any typical substrate 10, oftenformed from a silicon wafer. In other embodiments, the substrate 10 maycomprise other suitable materials (e.g., other group III-IV materials),or epitaxial layers formed over single crystal wafers.

With reference initially to FIG. 1, a thin, thermally-grown dielectriclayer (not shown), comprising pad oxide in a preferred embodiment, ispreferably first formed over the substrate 10. A hard mask layer 12,such as silicon nitride, may then be deposited over the substrate 10 anddielectric layer. The hard mask layer 12 may be formed by any well-knowndeposition process, such as sputtering, chemical vapor deposition (CVD)or spin-on deposition, among others. Although the hard mask layer 12comprises silicon nitride in a preferred embodiment, it must beunderstood that it may also be formed from silicon oxide, for example,or other materials that can protect underlying substrate during asubstrate etch and also withstand additional processing, as will beapparent from the fabrication steps described further below.

In a step also not illustrated in the figures, the hard mask layer 12may then be patterned using a photoresist layer formed over the hardmask layer 12. The photoresist layer may be patterned to form a maskusing conventional photolithographic techniques, and the hard mask layer12 may be anisotropically etched through the patterned photoresist toobtain a plurality of hard mask islands 14 within the array region ofthe memory device. The photoresist layer may then be removed byconventional techniques, such as by using an oxygen-based plasma. Inalternative embodiments, the hard mask layer 12 may be anisotropicallyetched to obtain a hard mask grid, which can generally provide similarfunctionality (namely, protecting portions of the substrate 10 that willserve to seed lateral epitaxial overgrowth) as the hard mask islands 14discussed at length below.

As shown in FIG. 1, which illustrates a cross-sectional view of aportion of the array, the silicon of the substrate 10 is thenselectively etched back. The etch process selectively etches thesubstrate 10 relative to the material forming the hard mask layer 12.For example, a selective wet etch may be used that strips siliconrelative to silicon nitride. In another embodiment, ion milling orreactive ion etching may be used. Thus, the array of the memory devicebecomes an array of silicon studs 16, preferably with one centered oneach active area region. Each of these silicon studs 16 is definedbeneath the hard mask islands 14. Meanwhile, at least a portion of theperiphery of the memory device is preferably left untouched, protectedby unpatterned regions of the hard mask 12.

In an alternative embodiment, only one silicon stud 16 need be formedfor a plurality of active areas. For example, one silicon stud 16 may beformed for every five active areas. However, in such an embodiment, someof the steps below, such as the lateral epitaxial growth of siliconshown in FIG. 3, may take much longer. As will be understood by theskilled artisan, steps not shown in the figures are typically performedto separate the active areas. For example, in one embodiment, a fieldoxide could be defined surrounding each active area to preventinterference between adjacent active areas. Separate field isolationsteps, on the other hand, may be omitted where localized or pseudo-SOIis formed individually for each active area, as illustrated.

In FIG. 2, the portions of the array stripped according to the stepsabove are shown filled by an insulator layer 18, preferably an oxide. Ina preferred embodiment, the insulator layer 18 is blanket deposited overthe array at least up to the height of the top surface of the siliconstuds 16. After deposition of a sufficient amount of insulator, theexcess that may have formed over the islands 14 and other portions ofthe device may be removed by any of a number of processes well-known tothose of skill in the art. For example, the surface of the device may beplanarized to the top surface of the hard mask islands 14, as shown inFIG. 2. Any suitable planarization process, such as, for example,chemical mechanical polishing (“CMP”), may be used.

Thus, the array preferably comprises a plurality of silicon studs 16surrounded by insulator 18, whereas the periphery will simply remain inits original configuration with a hard mask layer 12 overlying thedielectric layer (e.g., pad oxide, not shown) covering the substrate 10.

Turning to FIG. 3, another masking process, such as that describedabove, may be used within the array to open the insulator layer 18around the silicon studs 16, at least in the regions where active areasare desired. In the illustrated embodiment, each active area has its owntrench 20, such that unetched portions of the insulator 18 betweentrenches 20 serve as field isolation. As described above, this processis preferably performed using photoresist patterned according toconventional photolithographic techniques, optionally with a hard mask.In a preferred embodiment, a selective etching process may then be usedto selectively recess the insulator layer 18 relative to the hard masklayer 12 and substrate 10, thereby forming trenches 20 in the memorydevice surrounding the silicon studs 16. This etching process ispreferably continued until the trench 20 within the insulator layer 18achieves a depth greater than the height of the hard mask layer 12 butless than the height of the silicon studs 16, thereby exposing a portionof the silicon substrate 10 forming silicon studs 16. Preferably, thetrench 20 has a depth between about 200 Å and 1,000 Å.

In a preferred embodiment, a few layers of silicon 22 may then beepitaxially grown from the silicon stud 16, using the silicon as a seedlayer. As is well-known to those of skill in the art, the epitaxialgrowth produces silicon extensions 22 with the same crystallinestructure as the silicon substrate 10. Preferably, selective epitaxy isemployed to avoid the need for subsequent removal of polysilicon fromthe exposed oxide and nitride surfaces. Preferably between about 50 Åand 500 Å of silicon (or other semiconductor) is grown.

As shown in FIG. 4, a layer of amorphous silicon 24 may then bedeposited within the trench 20 formed around the silicon stud 16. In apreferred embodiment, the amorphous silicon 24 may be blanket depositedover the array, filling the trenches 20. After deposition of asufficient amount of silicon, the excess may be removed by any of anumber of processes well-known to those of skill in the art. As shown inFIG. 4, the surface of the device is preferably planarized to the topsurface of the hard mask islands 14. Any suitable planarization process,such as, for example, CMP, may be used. In another embodiment, thesilicon extensions 22 may instead be extended by epitaxial deposition inorder to fill the trenches 20. In still another arrangement,planarization may follow the crystallization step described below.

In a preferred embodiment, a thin oxide 23, shown in FIG. 4, may then begrown over the surface of the silicon layer 24, which may furtherfacilitate the crystallization of the filler silicon 24 using theepitaxially deposited silicon extension 22 as a seed layer.

The preferred silicon and oxide deposition is followed by an annealingprocess, whereby the amorphous silicon 24 has a tendency to take on acrystalline orientation similar to that of the epitaxially grown siliconextensions 22. Preferably, the amorphous silicon 24 takes an orderedcrystalline pattern. Such conversion is a species of solid phase epitaxy(SPE) known as epitaxial lateral overgrowth (ELO).

Finally, as shown in FIG. 5, the hard mask islands 14 may be removed,and the silicon layer 24 (preferably now crystallized) may be recessed.In a preferred embodiment, a selective etch may be used that etches thesilicon 24 and the hard mask layer 14 far more effectively than theinsulator layer 18, thereby exposing the silicon stud 16 for furtherprocessing steps. Further selective epitaxial deposition can then beconducted to achieve the desired thickness. Alternatively, the wholewafer can be planarized.

According to the above-described process, a localizedsilicon-over-insulator may be formed using relatively inexpensivefabrication techniques on a conventional polysilicon substrate. FIGS. 6and 7 show an arrangement where two memory cells share a singletransistor source. In particular, these figures illustrate capacitorlessDRAM formed on and within this SOI substrate. Of course, in otherembodiments, other DRAM schemes are also contemplated

In FIG. 6, a completed capacitorless DRAM structure is shown formed overlocalized SOI created according to the steps set forth above. Asillustrated, the silicon stud 16 remains beneath the common source andis connected by contacts 26 to a conductive digit or bit line 28. Thedrains 30 are located at the farthest ends of the crystallized siliconlayer 24 and are also electrically connected by contacts 32 with senselines 34. Floating bodies 36 form part of the channels that separate thedrains 30 and the source (at the top of the pillar 16) in the preferredembodiment, and these floating bodies 36 are directly adjacent an innerpair of word lines 38. This inner pair of word lines 38 preferablyseparates the source 16 and the drains 30, as may be seen in FIG. 6,serving as dual gates. While referred to hereinabove as drains 30 andsource 16, it will be understood that these are mere labels used forconvenience and for ready comparison to traditional capacitor-based DRAMdesigns. The labels can be reversed; whether the voltage is at a higherlevel at the source or the drain depends upon whether a read or a writeoperation is being performed, as described in more detail below.

The structure shown in FIG. 6 may be formed according to a number ofdeposition, pattern and etch steps well known to those of skill in theart. While configured for capacitorless DRAM operation, the illustratedscheme, whereby two memory cells share a common bit line 28 and bit linecontact 26, is otherwise similar to the scheme in U.S. Pat. No.6,660,584, issued to Tran, the disclosure of which is incorporatedherein by reference in its entirety. The '584 patent describes a “6F²”arrangement in which pairs of memory cells share a common bit line andsource region with independent pairs of word lines, drains andcapacitors. The process used to form the structure shown in FIG. 6 willdiffer, of course, from that in U.S. Pat. No. 6,660,584 to the extentthat the structure of FIG. 6 lacks capacitors.

Preferably, a gate oxide is first grown over the silicon layer, followedby a gate stack deposition and etching. The necessary doping implantsmay then be formed to define the source, drain and channel regions.Spacers 40 may be deposited and etched, in a typical spacer fabricationprocess well known to those of skill in the art, before some of thedoping steps. The bit line and cell side junctions are then formed,followed by formation of the metallic contacts and bit lines. Senseregions and other metallic contacts may then also be formed. Suchprocesses may be carried out in a number of ways, but the capacitorlessDRAM thus formed is particularly effective as a result of its formationover localized SOI. As a result of the SOI, the floating bodies 36function particularly well, isolated as they are within the insulatorlayer 18, and the devices of the periphery surrounding the array can betied to the bulk substrate 10.

In a preferred embodiment, the capacitorless DRAM shown in FIG. 6operates using gate-induced drain leakage (GIDL) current, although inother embodiments impact ionization current may also be used. As wouldbe well understood by those skilled in the art, capacitorless DRAM usesthe floating bodies 36 to store information regarding the state of thetransistor. In particular, in order to write a logical “1” value to thetransistor shown in FIG. 6, a “drain” 30 is placed at an elevatedvoltage relative to an adjacent gate (i.e., one of the word lines 38).The voltages of the drain 30 and gate 38 are controlled by the senselines 34 and the word lines 38 respectively. As a result of electrontunneling, electrons flow to the drain 30, while generated holes flow tothe floating bodies 36 underlying the gates.

As the holes accumulate in a floating body 36, the threshold voltage ofthe transistor is reduced, and the source current is thereby increased.Thus, a digital oscilloscope may be used, typically during the design ofthe capacitorless DRAM, to measure the source current and thereby thestate of the transistor. In the illustrated embodiment, this sourcecurrent may be detected along the raised bit line 28. In order to writea logical “0” value to one of the transistors, the adjacent gate takesan elevated voltage relative to the drain 30. Thus, the holes in thefloating body 36 are forced out, the threshold voltage increases again,and the source current is reduced. Again, a digital oscilloscope may beused to detect this change in source current in determining appropriateoperational thresholds. More information regarding how suchcapacitorless DRAM functions may be found in the article cited andincorporated above written by Eijiag Yoshida and Tetsu Tanaka.

As illustrated, each active area of the capacitorless DRAM forms part ofa pair of memory cells comprising two floating bodies 36, and atransistor having a single source 16 shared by the memory cells, twogates and two drains 30. The pair of memory cells, therefore, has twoaddressable locations, the floating bodies 36, that can each store onebit of data. This preferred embodiment functions generally as describedabove. However, in one application, the pair of memory cells may provideredundancy because, if either of the floating bodies 36 is storing a “1”bit, the source current at the bit line 28 is elevated. Thus, in oneembodiment, the read and write operations using the illustrated pair ofmemory cells will take place simultaneously to both floating bodies 36,thereby reducing errors.

Alternatively, the pair of memory cells may have three possible states.In one state, both floating bodies 36 store a “0” bit, and the sourcecurrent through the conductive line 28 is at its lowest level. In asecond state, one and only one of the floating bodies 36 stores a “1”bit, and the source current through the bit line 28 is at a higherlevel. Note that in this second state, the elevated source currentthrough the bit line 28 yields only the information that one of thefloating bodies 36 is storing a “1” bit, and does not indicate which ofthe floating bodies 36 is in this elevated state. In a third state, bothof the floating bodies 36 store a “1” bit, and the source currentthrough the bit line 28 is at its highest level. Thus, a sensitiveoscilloscope, for example, will be able to differentiate between thesethree states.

A schematic plan view of this capacitorless DRAM is shown in FIG. 7. Ofcourse, this capacitorless DRAM design is shown by way of example only,and the localized SOI method described above with reference to FIGS. 1-5may be used in any number of semiconductor environments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the invention. Indeed, the novel methodsand devices described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and devices described herein may be made withoutdeparting from the spirit of the invention. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the invention.

1. A method of forming capacitorless DRAM over localizedsilicon-on-insulator, the method comprising: providing a siliconsubstrate; defining an array of silicon studs within the siliconsubstrate; defining an insulator layer atop a portion of the siliconsubstrate, and between the silicon studs; defining a plurality ofsilicon-over-insulator layers atop the insulator layer, each of thesilicon-over-insulator layers surrounding a corresponding one of thesilicon studs; and forming a plurality of capacitorless DRAM cells, eachof the capacitorless DRAM cells formed within and above a correspondingone of the silicon-over-insulator layers.
 2. The method of claim 1,wherein defining the array of silicon studs comprises etching thesilicon substrate using photolithographic techniques.
 3. The method ofclaim 1, wherein defining the insulator layer comprises: depositing aninsulator material over the silicon substrate to a height at least equalto a height of the silicon studs; and planarizing the insulator materialand silicon substrate.
 4. The method of claim 1, wherein defining thesilicon-over-insulator layers comprises: etching at least a portion ofthe insulator layer between the silicon studs to define trenchessurrounding each silicon stud; and exposing at least some siliconmaterial comprising the silicon studs.
 5. The method of claim 4, whereindefining the silicon-over-insulator layers further comprises: depositingsilicon extensions adjacent sidewalls of each silicon stud by selectiveepitaxy.
 6. The method of claim 4, wherein defining thesilicon-over-insulator layers further comprises: depositing siliconwithin the trenches and converting the silicon by lateral epitaxialovergrowth.
 7. The method of claim 1, wherein forming the capacitorlessDRAM cells comprises: for each silicon-over-insulator layer, forming acommon source shared by a pair of memory cells, the memory cells furthercomprising two floating bodies, two gates and two drains.
 8. The methodof claim 7, wherein forming the capacitorless DRAM cells furthercomprises: forming the two floating bodies within eachsilicon-over-insulator layer.
 9. A method of forming a memory chip, themethod comprising: defining a periphery region and a memory array regionon the memory chip; forming at least one silicon-over-insulator regionin the memory array region, without forming a silicon-over-insulatorregion in at least part of the periphery region; and forming at leastone capacitorless DRAM cell on and within the at least onesilicon-over-insulator region.
 10. The method of claim 9, whereinforming the at least one capacitorless DRAM cell further comprisesdefining at least one floating body within the at least onesilicon-over-insulator region.
 11. The method of claim 10, whereinforming the at least one capacitorless DRAM cell further comprisesdefining a single, shared source in the memory array region for everytwo floating bodies.
 12. The method of claim 9, where forming at leastone silicon-over-insulator region comprises forming a silicon layer overan insulator by lateral epitaxial overgrowth.
 13. The method of claim12, wherein forming at least one silicon-over-insulator region furthercomprises forming a silicon pillar protruding through the insulatorprior to lateral epitaxial overgrowth.
 14. The method of claim 13,wherein forming at least one silicon-over-insulator region comprisesetching a recess with the pillar therein and depositing the insulatorinto the recess.
 15. The method of claim 13, further comprisingrecessing the insulator such that the pillar protrudes above theinsulator prior to lateral epitaxial overgrowth.
 16. The method of claim9, wherein forming the at least one capacitorless DRAM cell furthercomprises defining at least one floating body within the at least onesilicon-over-insulator region.
 17. The method of claim 16, whereinforming the at least one capacitorless DRAM cell further comprisesdefining a single, shared source in the memory array region for everytwo floating bodies.
 18. A method of forming capacitorless DRAM, themethod comprising: providing a silicon substrate; defining asilicon-over-insulator structure atop at least a portion of the siliconsubstrate wherein the silicon-over-insulator structure surrounds asilicon stud; and forming a capacitorless DRAM within and above thesilicon-over-insulator structure.
 19. The method of claim 18, whereinforming the capacitorless DRAM comprises defining two floating bodieswithin the silicon-over-insulator structure, wherein the two floatingbodies share a common source.
 20. The method of claim 18, whereindefining the silicon-over-insulator structure comprises forming siliconepitaxial extensions laterally over a deposited insulator by laterallyextending single crystal silicon from the silicon stud.
 21. A method offorming a memory chip, the method comprising: defining an active regionon the memory chip; forming a silicon pillar in the active region;filling a trench surrounding the silicon pillar with an insulatinglayer; recessing the insulating layer to expose an upper portion of thesilicon pillar; forming epitaxial silicon over the recessed insulatinglayer around the silicon pillar; and forming at least one capacitorlessDRAM cell on and within the epitaxial silicon.
 22. The method of claim21, wherein forming epitaxial silicon comprises growing about 50 Å and500 Å of silicon over the recessed insulating layer.
 23. The method ofclaim 22, further comprising: defining a floating body within theepitaxial silicon; defining a source at the top of the silicon pillarfor the floating body, wherein the floating body separates the sourcefrom a drain; and defining a bit line connected to the common source.24. A method of operating a capacitorless DRAM, the method comprising:placing a floating body in a first state; and detecting the first stateby measuring a first current at a source of the capacitorless DRAM;wherein the floating body is defined within a localizedsilicon-over-insulator.